German Offenlegungsschrift (Laid-Open Patent) 3,843,516 shows such an apparatus. Such plants operate with a vacuum. In this case the vacuum is produced there by a single vacuum pump 43 (bottom right in FIG. 1) of the German Laid-Open Patent. The vacuum is maintained through duct 42. The vacuum does not remain constantly in existence. This is less because of air leaks but because a beverage, e.g. in the form of wine or beer, is in fact supplied through the duct 11. At least to this extent, in that a supply of liquid takes place here, but also because of thermal expansion and for other reasons, it is not sufficient to produce a vacuum once only. Rather, the vacuum must be continuously maintained. The word "vacuum" is not to be taken literally here, since an absolute vacuum cannot be produced economically. One or more vacuum pumps can be made use of for the production of this vacuum. Water ring vacuum pumps are usually used. However, rotary vane vacuum pumps can be used, or even venturi nozzles, with which a lower degree of pressure reduction can be attained in a cheaper manner than with water ring vacuum pumps.
Wine, cider, home-made wine, perry, etc., have 400 volatile aroma materials. The majority of these wines have H.sub.2 SO.sub.3 (sulfurous acid), which is also necessary and permissible, so that oxidation is prevented.
The vacuum pumps now have the property of drawing off volatile constituents, e.g. via the duct 42 of the state of the art, and to discharge them through their exhaust air pipes or the like into the surroundings. On smelling these exhaust air pipes, they are found to smell of sulfur and/or aroma materials. The loading of the environment is very small. However, the final product lacks these materials. Indeed, during the reduction of the alcohol content, in the ideal case it would be preferred to remove solely the ethyl alcohol and not the aroma materials. A filter would be desired with an infinite edge steepness for very selective filtration. Moreover, the liquid of reduced alcohol content is also to be stable, so that it has a sufficiently long life.